1. Field of the Invention
This invention relates to a connectible instrument casing. More particularly, the invention provides a connectible instrument casing which is suitable for easy assembling of a number of instruments at a low cost so as to form a multi-loop control system including a plurality of instruments and/or distributed automatic controllers. The instrument here refers to an indicating meter, a recording meter, a display, a switching means, an automatic controller with or without a meter, and the like.
2. Description of the Prior Art
In controlling the process of a typical plant, the overall process control system usually includes a plurality of control loops, and the case of having only one control loop is rather rare. The number of control loops in the process control system varies considerably depending on the kind and scale of the plant. The number of automatic controllers per control loop is not necessarily one. For instance, some cascade control systems use one automatic controller per two control loops. In general, the number of meters and automatic controllers in a process control system varies over a wide range depending on the scale and kind of the plant concerned, and the type of control also varies extensively plant by plant.
In the instrumentation and control system of the prior art, each of the meters required is placed in a separate casing and mounted on a panel one by one, or holes are cut on the panel so as to mount the required number of meters on the thus cut holes one by one. The need of separate casings for the individual meters has been one of causes to increase the cost of the conventional control system and instrumentation.
To avoid this difficulty, a common casing for a certain number of instruments has been proposed. For instance, FIG. 1 shows a common casing 52 for six instruments 51. However, such common casing for a predetermined number of instruments has a shortcoming in that, if the number of instruments to be housed in the casing exceeds the predetermined number only by a few, for instance, if seven or eight instruments 51 are to be housed in the casing 52 of FIG. 1, remodelling of the casing may become necessary. On the other hand, if less than the predetermined number of instruments are housed in such common casing, for instance, if only three instruments 51 are to be housed in the common casing 52 of FIG. 1, it becomes necessary to mount blind covers onto the unused portion of the casing 52. Such remodelling or mounting of the blind covers tends to cause an extra cost.